Portal has a series of specific Payloads that it attaches to a VAA from a source chain to tell the target chain what to do with the Wormhole message when it’s verified.
Tokens are transferred from one chain to another using a lockup/mint and burn/unlock mechanism. While many bridges work on this basic premise, this implementation achieves this by relying on the generic message passing protocol provided by Wormhole to support routing the lock and burn events from one chain to another. This makes Wormhole's token bridge completely chain agnostic. As long as a wormhole contract exists on the chain we wish to transfer to, an implementation can be quickly incorporated into the network. Due to the generic message passing nature of Wormhole, programs emitting messages do not need to know anything about the implementation details of any other chain.
In order to transfer tokens from A to B, we must lock the tokens on A and mint them on B. It is important that the tokens on A have been proven to be locked before the minting can occur on B. To facilitate this process, chain A first locks the tokens and emits a message indicating that the locking has been completed. This message has the following structure, and is referred to as a transfer message:
u8 payload_id = 1 Transfer
u256 amount Amount of tokens being transferred.
u8 token_address Address on the origin chain.
u16 token_chain Numeric ID for the origin chain.
u8 to Address on the destination chain.
u16 to_chain Numeric ID for the destination chain.
u256 fee Portion of amount paid to a relayer.
This structure contains contains everything needed for the receiving chain to learn about a lockup event. Once Chain B receives this payload it can mint the corresponding asset.
Note that Chain B is agnostic as to how the tokens on the sending side were locked. They could have been burned by a mint or locked in a custody account. The protocol simply relays the event once a sufficient amount of guardians have attested its existence.
The Transfer event above is missing an important detail. While the program on Chain B can trust the message to inform it of token lockup events, it has no way to know what the token being locked up actually is. The address alone is a meaningless value to most users. To solve this, the Token Bridge supports token attestation. Chain A emits a message containing metadata about an address, which Chain B can store in order to learn the name, symbol, and decimal precision of a token address. The message format for this action is as so:
u8 payload_id = 2
An important detail of the token bridge is that an attestation is in fact required before a token can be transferred. This is because without knowing a tokens decimal precision, it is not possible for Chain B to correctly mint the correct amount of tokens when processing a transfer.****